Polylactic acid is known as one of bio-based degradable polymers with great potential of application. Polylactic acid can be applied in bioengineering, health care and other fields for its good biocompatibility and biodegradability. Particularly, polylactic acid fiber membranes can be used for a cosmetic (e.g. a facial mask), a drug controlled release carrier, a tissue repair bracket, an anti-adhesion membrane for operation, a wound membrane, artificial tissues or organ template (artificial bone, artificial nerve conduit, artificial blood vessel, etc.) and so on, and can be used for filtering and absorbing heavy metals and other toxic substances as well. For these applications, polylactic acid fiber membranes are required to have low enough fiber diameters to varying degrees, optimally nano-scale, for increase in specific surface area and reduction of the pore diameter of the fiber membranes. Additionally, for packing, transportation, storage, sterilization, etc., polylactic acid fiber membranes are required to have high enough thermal resistance and optimally maintain stability of dimension and performance during medical sterilization at a temperature above 120° C.
Most of developed polylactic acid nano-fiber membranes are prepared via the electro-spinning method. Chinese patent application with Publication Number of CN1994476A discloses a medical polylactic acid composite nano-membrane prepared via the electro-spinning method, and sterilized via the Co-60 radiation sterilization method; Chinese patent application with Publication Number of CN101327345A discloses a polylactic acid/polyhydroxyalkanoate compounding fiber membrane prepared with the electro-spinning method, wherein the fiber diameter is 20 to 600 nm and the pore diameter of the porous membrane is 100 nm to 10 μm; Chinese patent application with Publication Number of CN101401955A discloses a levorotatory polylactic acid nano-fiber membrane prepared via the electro-spinning method, wherein the fiber diameter is 50 to 500 nm and the porosity factor of membrane is more than 90%; and Chinese patent application with Publication Number of CN102086565A discloses a polylactic acid anti-microbial nano-fiber membrane and preparation method thereof, wherein the polylactic acid anti-microbial nano-fiber membrane is prepared by adding an antibacterial agent to the polylactic acid with the electro-spinning method.
Although polylactic acid nano-fiber membranes can be prepared with the above disclosed technologies, the following limitations exist: (1) under the current technical conditions, the electro-spinning has low production efficiency and complicated process requirements; and (2) a method for improving thermal resistance of the polylactic acid membrane has not been put forward. It is well known that poor thermal resistance is one of the major defects of polylactic acid. Polylactic acid is also known in the art as polylactide. A conventional polylactic acid fiber has more than 10% boiling water shrinkage, which results in serious deformation of the polylactic acid fiber when temperature is more than 100° C., with great restrictions on its application.
It has been revealed that poly(L-lactide) (PLLA) doped with enantiomeric poly(D-lactide) (PDLA) could form a racemic stereocomplex crystal by compounding crystallization. Since the stereocomplex crystal shows a melting point of 40 to 70° C. higher than that of homocrystalline polylactic acid, it is highly expected to improve thermal resistance of polylactic acid products by forming the racemic stereocomplex crystal. Takasaki et al. (J. Macromol. Sci., Pt.B-Phys. 2003, B42:403) prepared a compounding fiber containing equal amounts of poly(L-lactide) and poly(D-lactide) molecules with a melt spinning method. Although the racemic stereocomplex crystal is formed at take-up velocity of 1000 to 7500 m/min, the fiber showed more than 10% shrinkage at 100° C., indicating that its thermal resistance was unsatisfactory.
Therefore, forming a racemic stereocomplex crystal in polylactic acid is not a sufficient condition for a significant improvement of its thermal resistance. In other words, the heat resistance of polylactic acid products such as nano-fiber membranes is not necessarily to be improved even though a racemic stereocomplex crystal is formed. At the same time, a racemic stereocomplex crystal formed with the known technologies requires that poly(L-lactide) is present in the same or similar amounts than poly(D-lactide), namely poly(D-lactide) is about 50% of the raw materials. However, poly(D-lactide) is much more expensive than the poly(L-lactide), so such methods are uneconomic due to significant increase in material cost. Therefore, it is necessary to develop a new polylactic acid nano-fiber membrane and a preparation method thereof to meet the higher requirements for its overall performance during its application, including heat resistance.